Laser Beam Machining Method And Semiconductor Chip

ABSTRACT

A laser processing method is provided, which, when cutting a substrate formed with a multilayer part including a plurality of functional devices, makes it possible to cut the multilayer part with a high precision in particular. 
     In a state where a protective tape  22  is attached to the front face  16   a  of a multilayer part  16 , a substrate  4  is irradiated with laser light L while using its rear face  4   b  as a laser light entrance surface, so as to form a modified region  7  within the substrate  4  along a line to cut, thereby generating a fracture  24  reaching the front face  4   a  of the substrate  4  from a front-side end part  7   a  of the modified region  7 . Attaching an expandable tape to the rear face  4   b  of the substrate  4  and expanding it in the state where such a fracture  24  is generated can cut not only the substrate  4  but also the multilayer part  16  on the line to cut, i.e., interlayer insulating films  17   a   , 17   b , with a favorable precision along the line to cut.

TECHNICAL FIELD

The present invention relates to a laser processing method used forcutting a substrate formed with a multilayer part including a pluralityof functional devices, and a semiconductor chip cut by using such alaser processing method.

BACKGROUND ART

Conventionally known as this kind of technique is a laser processingmethod which irradiates a substrate formed with a multilayer partincluding a plurality of functional devices while locating a convergingpoint therewithin, so as to form a modified region within the substratealong a line to cut, and cuts the substrate and multilayer part from themodified region acting as a start point (see, for example, PatentDocument 1).

Patent Document 1: Japanese Patent Application Laid-Open No. 2003-334812DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The laser processing method such as the one mentioned above is aneffective technique in that it can cut the substrate and multilayer partwith a high precision. In connection with such a technique, there hasbeen a demand for a technique which, when forming a modified regionwithin a substrate formed with a multilayer part including a pluralityof functional devices along a line to cut, makes it possible to cut themultilayer part with a higher precision in particular from the modifiedregion acting as a start point.

In view of such circumstances, it is an object of the present inventionto provide a laser processing method which, when cutting a substrateformed with a multilayer part including a plurality of functionaldevices, makes it possible to cut the multilayer part with a highprecision in particular, and a semiconductor chip cut by using such alaser processing method.

For achieving the above-mentioned object, the laser processing method inaccordance with the present invention is a laser processing method ofirradiating a substrate having a front face formed with a multilayerpart including a plurality of functional devices with laser light whilelocating a converging point within the substrate, so as to form amodified region to become a starting point region for cutting within thesubstrate along a line to cut in the substrate, wherein the modifiedregion is formed such as to generate a fracture reaching at least thefront face of the substrate from a front-side end part of the modifiedregion.

This laser processing method forms a modified region within thesubstrate along a line to cut such as to generate a fracture reaching atleast the front face of the substrate from a front-side end part of themodified region. Attaching an expandable member such as expandable tape,for example, to the rear face of the substrate and expanding it while inthe state generating such a fracture can cut not only the substrate butalso the multilayer part along the line to cut with a favorableprecision in particular. Therefore, when cutting a substrate formed witha multilayer part including a plurality of functional devices, thislaser processing method makes it possible to cut the multilayer partwith a high precision in particular.

The functional device refers to semiconductor operating layers formed bycrystal growth, light-receiving devices such as photodiodes,light-emitting devices such as laser diodes, and circuit devices formedas circuits, for example. The modified region is formed by irradiatingthe substrate with laser light while locating a converging pointtherewithin, so as to form multiphoton absorption or light absorptionequivalent thereto within the substrate.

The above-mentioned laser processing method may generate either afracture reaching the inside of the multilayer part from the front-sideend part of the modified region or a fracture reaching the front face ofthe multilayer part from the front-side end part of the modified region.

In another aspect, the laser processing method in accordance with thepresent invention is a laser processing method of irradiating asubstrate having a front face formed with a multilayer part including aplurality of functional devices with laser light while locating aconverging point within the substrate, so as to form a modified regionto become a starting point region for cutting within the substrate alonga line to cut in the substrate, wherein the modified region is formedsuch that the front-side end part of the modified region extends like astreak to the front face of the substrate.

This laser processing method forms a modified region within thesubstrate along a line to cut such that the front-side end part of themodified region extends like a streak to the front face of thesubstrate. Thus forming the modified region generates a fracturereaching at least the front face of the substrate from the front-sideend part of the modified region. Attaching an expandable member such asexpandable tape, for example, to the rear face of the substrate andexpanding it while in the state generating such a fracture can cut notonly the substrate but also the multilayer part along the line to cutwith a favorable precision in particular. Therefore, when cutting asubstrate formed with a multilayer part including a plurality offunctional devices, this laser processing method makes it possible tocut the multilayer part with a high precision in particular.

Here is a case where the substrate is a semiconductor substrate, whilethe modified region includes a molten processed region. Since thismolten processed region is an example of the above-mentioned modifiedregion, the multilayer part can be cut with a high precision inparticular when cutting the substrate formed with the multilayer partincluding a plurality of functional devices in this case as well.

There is also a case where the substrate is a semiconductor substrate,while the modified region includes a molten processed region and amicrocavity positioned closer to the front face of the substrate than isthe molten processed region. Since each of the molten processed regionand microcavity is an example of the above-mentioned modified region,the multilayer part can be cut with a high precision in particular whencutting the substrate formed with the multilayer part including aplurality of functional devices in this case as well.

Preferably, the substrate has a thickness of 30 μm to 150 μm. When thethickness of the substrate is 30 μm to 150 μm, not only the multilayerpart but also the substrate can be cut with a high precision from theabove-mentioned modified region as a start point.

The substrate and multilayer part may be cut along the line to cut afterforming the modified region. In this case, when cutting the substrateformed with the multilayer part including a plurality of functionaldevices, the multilayer part can be cut along the line to cut with afavorable precision in particular from the reason mentioned above.

In still another aspect, the laser processing method in accordance withthe present invention is a laser processing method of irradiating asubstrate having a front face formed with a multilayer part including aplurality of functional devices with laser light while locating aconverging point within the substrate, so as to form a modified regionto become a starting point region for cutting within the substrate alonga line to cut in the substrate, wherein the modified region is formedsuch that the distance between a position of a front-side end part ofthe modified region and the front face of the substrate is 3 μm to 40μm.

This laser processing method forms the modified region within thesubstrate along the line to cut such that the distance between theposition of the front-side end part of the modified region and the frontface of the substrate is 3 μm to 40 μm. Thus forming the modified regiongenerates a fracture reaching at least the front face of the substratefrom the front-side end part of the modified region. Attaching anexpandable member such as expandable tape, for example, to the rear faceof the substrate and expanding it while in the state generating such afracture can cut not only the substrate but also the multilayer partalong the line to cut with a favorable precision in particular.Therefore, when cutting a substrate formed with a multilayer partincluding a plurality of functional devices, this laser processingmethod makes it possible to cut the multilayer part with a highprecision in particular.

In the case where the laser light is irradiated once along the line tocut in the above-mentioned laser processing method, it will be preferredif the modified region is formed such that the distance between theposition of the front-side end part of the modified region and the frontface of the substrate is 3 μm to 35 μm. In the case where the laserlight is irradiated a plurality of times along the line to cut in theabove-mentioned laser processing method, it will be preferred if themodified region is formed such that the distance between the position ofthe front-side end part of the modified region and the front face of thesubstrate is 3 μm to 40 μm. Forming the modified region under such acondition can reliably generate a fracture reaching at least the frontface of the substrate from the front-side end part of the modifiedregion.

The semiconductor chip in accordance with the present invention is asemiconductor chip comprising a substrate and a multilayer part, formedon a front face of the substrate, including a functional device, a sideface of the substrate being formed with a modified region, wherein themodified region is formed such that the distance between a position of afront-side end part of the modified region and the front face of thesubstrate is 3 μm to 40 μm.

This semiconductor chip can be construed as one cut by using theabove-mentioned laser processing method, whereby an end part of themultilayer part corresponding to the side face of the substrate formedwith the modified region is one cut with a high precision.

EFFECT OF THE INVENTION

When cutting a substrate formed with a multilayer part including aplurality of functional devices, the present invention makes it possibleto cut the multilayer part with a high precision in particular.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A plan view of an object to be processed during laserprocessing by a laser processing method in accordance with anembodiment.

[FIG. 2] A sectional view of the object taken along the line II-II ofFIG. 1.

[FIG. 3] A plan view of the object after the laser processing by thelaser processing method in accordance with the embodiment.

[FIG. 4] A sectional view of the object taken along the line IV-IV ofFIG. 3.

[FIG. 5] A sectional view of the object taken along the line V-V of FIG.3.

[FIG. 6] A plan view of the object cut by the laser processing method inaccordance with the embodiment.

[FIG. 7] A graph showing relationships between the field intensity andcrack spot size in the laser processing method in accordance with theembodiment.

[FIG. 8] A sectional view of the object in a first step of the laserprocessing method in accordance with the embodiment.

[FIG. 9] A sectional view of the object in a second step of the laserprocessing method in accordance with the embodiment.

[FIG. 10] A sectional view of the object in a third step of the laserprocessing method in accordance with the embodiment.

[FIG. 11] A sectional view of the object in a fourth step of the laserprocessing method in accordance with the embodiment.

[FIG. 12] A view showing a photograph of a cut section in a part of asilicon wafer cut by the laser processing method in accordance with theembodiment.

[FIG. 13] A graph showing relationships between the laser lightwavelength and the transmittance within a silicon substrate in the laserprocessing method in accordance with the embodiment.

[FIG. 14] A sectional view of a silicon wafer formed with a moltenprocessed region and a microcavity by the laser processing method inaccordance with the embodiment.

[FIG. 15] A sectional view of a silicon wafer for explaining a principleby which the molten processed region and microcavity are formed by thelaser processing method in accordance with the embodiment.

[FIG. 16] A view showing photographs of a cut section of a silicon waferformed with molten processed regions and microcavities by the laserprocessing method in accordance with the embodiment.

[FIG. 17] A plan view of the object to be processed in the laserprocessing method in accordance with the embodiment.

[FIG. 18] A sectional view of a part of the object taken along the lineXVIII-XVIII of FIG. 17.

[FIG. 19] A sectional view of a part of the object for explaining thelaser processing method in accordance with the embodiment, in which (a)is a state where a protective tape is attached to the object, and (b) isa state where the object is irradiated with laser light.

[FIG. 20] A sectional view of a part of the object for explaining thelaser processing method in accordance with the embodiment, in which (a)is a state where an expandable tape is attached to the object, and (b)is a state where the protective tape is irradiated with UV rays.

[FIG. 21] A sectional view of a part of the object for explaining thelaser processing method in accordance with the embodiment, in which (a)is a state where the protective tape is peeled off from the object, and(b) is a state where the expandable tape is expanded.

[FIG. 22] A sectional view of a part of the object in which a crackreaching the inside of a multilayer part from the front-side end part ofa modified region is generated.

[FIG. 23] A sectional view of a part of the object in which a crackreaching the front face of the multilayer part from the front-side endpart of the modified region is generated.

[FIG. 24] A sectional view of a part of the object for explaining afirst reason why a low dielectric constant film can be cut with a highprecision when a fracture reaching the front face of the substrate fromthe front-side end part of the modified region is generated.

[FIG. 25] A sectional view of a part of the object for explaining asecond reason why a low dielectric constant film can be cut with a highprecision when a fracture reaching the front face of the substrate fromthe front-side end part of the modified region is generated.

[FIG. 26] A sectional view of a part of the object for explaining areason why a low dielectric constant film can be cut with a highprecision when a fracture reaching the front face of the low dielectricconstant film from the front-side end part of the modified region isgenerated.

[FIG. 27] A view showing photographs representing results of cutting ofthe object in the case where the fracture has reached the front face ofthe substrate and the case where the fracture has reached the front faceof the low dielectric constant film.

[FIG. 28] A chart showing “the relationship between the distance fromthe position of the front-side end part of the modified region to thefront face of the substrate and the state of the substrate” concerning asubstrate having a thickness of 30 μm.

[FIG. 29] A chart showing “the relationship between the distance fromthe position of the front-side end part of the modified region to thefront face of the substrate and the state of the substrate” concerning asubstrate having a thickness of 50 μm.

[FIG. 30] A chart showing “the relationship between the distance fromthe position of the front-side end part of the modified region to thefront face of the substrate and the state of the substrate” concerning asubstrate having a thickness of 100 μm.

[FIG. 31] A chart showing “the relationship between the distance fromthe position of the front-side end part of the modified region to thefront face of the substrate and the state of the substrate” concerning asubstrate having a thickness of 150 μm.

[FIG. 32] A view showing photographs of a cut section of a substrate inwhich a modified region is formed such that the front-side end part ofthe modified region extends like a streak to the front face of thesubstrate.

EXPLANATIONS OF NUMERALS AND LETTERS

1 . . . object to be processed; 4 . . . substrate; 4 a . . . front faceof the substrate; 4 b . . . rear face of the substrate; 4 c . . . sideface of the substrate; 5 . . . line to cut; 7 . . . modified region; 7 a. . . front-side end part of the modified region; 13 . . . moltenprocessed region; 14 . . . microcavity; 15 . . . functional device; 16 .. . multilayer part; 16 a . . . front face of the multilayer part; 22 .. . protective tape (protective member); 23 . . . expandable tape(expandable member); 24 . . . fracture; 25 . . . semiconductor chip; 26. . . low dielectric constant film; L . . . laser light; P . . .converging point

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, a preferred embodiment of the present invention willbe explained in detail with reference to the drawings. In the laserprocessing method in accordance with the embodiment, a phenomenon knownas multiphoton absorption is used for forming a modified region withinan object to be processed. Therefore, to begin with, a laser processingmethod for forming a modified region by the multiphoton absorption willbe explained.

A material becomes transparent when its absorption bandgap E_(G) isgreater than photon energy hv. Consequently, a condition under whichabsorption occurs in the material is hv>E_(G). However, even whenoptically transparent, the material generates absorption under acondition of nhv>E_(G) (where n=2, 3, 4, . . . ) if the intensity oflaser light becomes very high. This phenomenon is known as multiphotonabsorption. In the case of pulsed waves, the intensity of laser light isdetermined by the peak power density (W/cm²) of laser light at itsconverging point. The multiphoton absorption occurs under a conditionwhere the peak power density is 1×10⁸ (W/cm²) or greater, for example.The peak power density is determined by (energy of laser light at theconverging point per pulse)/(beam spot cross-sectional area of laserlight×pulse width). In the case of continuous waves, the intensity oflaser light is determined by the field intensity (W/cm²) of laser lightat the converging point.

The principle of the laser processing method in accordance with theembodiment using such multiphoton absorption will be explained withreference to FIGS. 1 to 6. As shown in FIG. 1, on a front face 3 of aplanar object to be processed 1, a line to cut 5 for cutting the object1 exists. The line to cut 5 is a virtual line extending straight. Asshown in FIG. 2, the laser processing method in accordance with thisembodiment irradiates the object 1 with laser light L while locating aconverging point P therewithin under a condition generating multiphotonabsorption, so as to form a modified region 7. The converging point P isa position at which the laser light L is converged. The line to cut 5may be curved instead of being straight, and may be a line actuallydrawn on the object 1 without being restricted to the virtual line.

Then, the laser light L is relatively moved along the line to cut 5(i.e., in the direction of arrow A in FIG. 1), so as to shift theconverging point P along the line to cut 5. Consequently, as shown inFIGS. 3 to 5, the modified region 7 is formed along the line to cut 5within the object 1, and becomes a starting point region for cutting 8.The starting point region for cutting 8 refers to a region which becomesa start point for cutting (fracturing) when the object 1 is cut. Thestarting point region for cutting 8 may be made by forming the modifiedregion 7 either continuously or intermittently.

In the laser processing method in accordance with this embodiment, themodified region 7 is not formed by the heat generated from the object 1absorbing the laser light L. The laser light L is transmitted throughthe object 1, so as to generate multiphoton absorption therewithin,thereby forming the modified region 7. Therefore, the front face 3 ofthe object 1 hardly absorbs the laser light L and does not melt.

Forming the starting point region for cutting 8 within the object 1makes it easier to generate fractures from the starting point region forcutting 8 acting as a start point, whereby the object 1 can be cut witha relatively small force as shown in FIG. 6. Therefore, the object 1 canbe cut with a high precision without generating unnecessary fractures onthe front face 3 of the object 1.

There seem to be the following two ways of cutting the object 1 from thestarting point region for cutting 8 acting as a start point. One iswhere an artificial force is applied to the object 1 after the startingpoint region for cutting 8 is formed, so that the object 1 fracturesfrom the starting point region for cutting 8 acting as a start point,whereby the object 1 is cut. This is the cutting in the case where theobject 1 has a large thickness, for example. Applying an artificialforce refers to exerting a bending stress or shear stress to the object1 along the starting point region for cutting 8, or generating a thermalstress by applying a temperature difference to the object 1, forexample. The other is where the forming of the starting point region forcutting 8 causes the object 1 to fracture naturally in itscross-sectional direction (thickness direction) from the starting pointregion for cutting 8 acting as a start point, thereby cutting the object1. This becomes possible if the starting point region for cutting 8 isformed by one row of the modified region 7 when the object 1 has a smallthickness, or if the starting point region for cutting 8 is formed by aplurality of rows of the modified region 7 in the thickness directionwhen the object 1 has a large thickness. Even in this naturallyfracturing case, fractures do not extend onto the front face 3 at aportion corresponding to an area not formed with the starting pointregion for cutting 8 in the part to cut, so that only the portioncorresponding to the area formed with the starting point region forcutting 8 can be cleaved, whereby cleavage can be controlled well. Sucha cleaving method with a favorable controllability is very effective,since the object 1 to be processed such as silicon wafer has recentlybeen apt to decrease its thickness.

The modified region formed by multiphoton absorption in the laserprocessing method in accordance with this embodiment encompasses thefollowing cases (1) to (4):

(1) Case where the Modified Region is a Crack Region Including One Crackor a Plurality of Cracks

An object to be processed (e.g., glass or a piezoelectric material madeof LiTaO₃) is irradiated with laser light while locating a convergingpoint therewithin under a condition with a field intensity of at least1×10⁸ (W/cm²) at the converging point and a pulse width of 1 μs or less.This magnitude of pulse width is a condition under which a crack regioncan be formed only within the object while generating multiphotonabsorption without causing unnecessary damages on the front face of theobject. This generates a phenomenon of optical damage by multiphotonabsorption within the object. This optical damage induces a thermaldistortion within the object, thereby forming a crack regiontherewithin. The upper limit of field intensity is 1×10¹² (W/cm²), forexample. The pulse width is preferably 1 ns to 200 ns, for example. Theforming of a crack region by multiphoton absorption is disclosed, forexample, in “Internal Marking of Glass Substrate with Solid-stateLaser”, Proceedings of the 45th Laser Materials Processing Conference(December, 1998), pp. 23-28.

The inventors determined the relationship between field intensity andcrack size by an experiment. The following are conditions of theexperiment.

(A) Object to be Processed: Pyrex (Registered Trademark) Glass (with aThickness of 700 μm)

(B) Laser

-   -   light source: semiconductor laser pumping Nd:YAG laser    -   wavelength: 1064 nm    -   laser light spot cross-sectional area: 3.14×10⁻⁸ cm²    -   oscillation mode: Q-switched pulse    -   repetition frequency: 100 kHz    -   pulse width: 30 ns    -   output: output<1 mJ/pulse    -   laser light quality: TEM₀₀    -   polarizing property: linear polarization

(C) Condenser Lens

-   -   transmittance at a laser light wavelength: 60%

(D) Moving Rate of the Mount Table Mounting the Object: 100 mm/sec

The laser light quality of TEM₀₀ means that the convergingcharacteristic is so high that convergence to about the wavelength oflaser light is possible.

FIG. 7 is a graph showing the results of the above-mentioned experiment.The abscissa indicates the peak power density. Since the laser light ispulsed laser light, the field intensity is represented by the peak powerdensity. The ordinate indicates the size of a crack part (crack spot)formed within the object by one pulse of laser light. Crack spots gatherto yield a crack region. The crack spot size is the size of a partyielding the maximum length among forms of crack spots. Data representedby black circles in the graph refer to a case where the condenser lens(C) has a magnification of ×100 and a numerical aperture (NA) of 0.80.On the other hand, data represented by whitened circles in the graphrefer to a case where the condenser lens (C) has a magnification of ×50and a numerical aperture (NA) of 0.55. Crack spots are seen to occurwithin the object from when the peak power density is about 10¹¹ (W/cm²)and become greater as the peak power density increases.

A mechanism by which the object to be processed is cut by forming acrack region will now be explained with reference to FIGS. 8 to 11. Asshown in FIG. 8, the object 1 is irradiated with laser light L while theconverging point P is located within the object 1 under a conditionwhere multiphoton absorption occurs, so as to form a crack region 9therewithin along a line to cut. The crack region 9 is a regioncontaining one crack or a plurality of cracks. Thus formed crack region9 becomes a starting point region for cutting. A crack further growsfrom the crack region 9 acting as a start point (i.e., from the startingpoint region for cutting acting as a start point) as shown in FIG. 9,and reaches the front face 3 and rear face 21 of the object 1 as shownin FIG. 10, whereby the object 1 fractures and is consequently cut asshown in FIG. 11. The crack reaching the front face 3 and rear face 21of the object 1 may grow naturally or as a force is applied to theobject 1.

(2) Case where the Modified Region is a Molten Processed Region

An object to be processed (e.g., semiconductor material such as silicon)is irradiated with laser light while locating a converging point withinthe object under a condition with a field intensity of at least 1×10⁸(W/cm²) at the converging point and a pulse width of 1 μs or less. As aconsequence, the inside of the object is locally heated by multiphotonabsorption. This heating forms a molten processed region within theobject. The molten processed region encompasses regions once molten andthen re-solidified, regions just in a molten state, and regions in theprocess of being re-solidified from the molten state, and can also bereferred to as a region whose phase has changed or a region whosecrystal structure has changed. The molten processed region may also bereferred to as a region in which a certain structure changes to anotherstructure among monocrystal, amorphous, and polycrystal structures. Forexample, it means a region having changed from the monocrystal structureto the amorphous structure, a region having changed from the monocrystalstructure to the polycrystal structure, or a region having changed fromthe monocrystal structure to a structure containing amorphous andpolycrystal structures. When the object to be processed is of a siliconmonocrystal structure, the molten processed region is an amorphoussilicon structure, for example. The upper limit of field intensity is1×10¹² (W/cm²), for example. The pulse width is preferably 1 ns to 200ns, for example.

By an experiment, the inventors verified that a molten processed regionwas formed within a silicon wafer. The following are conditions of theexperiment.

(A) Object to be Processed: Silicon Wafer (with a Thickness of 350 μmand an Outer Diameter of 4 Inches)

(B) Laser

-   -   light source: semiconductor laser pumping Nd:YAG laser    -   wavelength: 1064 nm    -   laser light spot cross-sectional area: 3.14×10⁻⁸ cm²    -   oscillation mode: Q-switched pulse    -   repetition frequency: 100 kHz    -   pulse width: 30 ns    -   output: 20 μJ/pulse    -   laser light quality: TEM₀₀    -   polarizing property: linear polarization

(C) Condenser Lens

-   -   magnification: ×50    -   N.A.: 0.55    -   transmittance at a laser light wavelength: 60%

(D) Moving Rate of the Mount Table Mounting the Object: 100 mm/sec

FIG. 12 is a view showing a photograph of a cross section of a part of asilicon wafer cut by laser processing under the conditions mentionedabove. A molten processed region 13 is formed within the silicon wafer11. The molten processed region 13 formed under the above-mentionedconditions has a size of about 100 μm in the thickness direction.

The fact that the molten processed region 13 is formed by multiphotonabsorption will now be explained. FIG. 13 is a graph showingrelationships between the laser light wavelength and the transmittancewithin the silicon substrate. Here, the respective reflected componentson the front and rear sides of the silicon substrate are eliminated, soas to show the internal transmittance alone. The respectiverelationships are shown in the cases where the thickness t of thesilicon substrate is 50 μm, 100 μm, 200 μm, 500 μm, and 1000 μm.

For example, at the Nd:YAG laser wavelength of 1064 nm, the laser lightappears to be transmitted through the silicon substrate by at least 80%when the silicon substrate has a thickness of 500 μm or less. Since thesilicon wafer 11 shown in FIG. 12 has a thickness of 350 μm, the moltenprocessed region 13 caused by multiphoton absorption is formed near thecenter of the silicon wafer 11, i.e., at a part distanced from the frontface by 175 μm. The transmittance in this case is 90% or more withreference to a silicon wafer having a thickness of 200 μm, whereby thelaser light is absorbed only slightly within the silicon wafer 11 but issubstantially transmitted therethrough. This means that the moltenprocessed region 13 is formed within the silicon wafer 11 not by laserlight absorption within the silicon wafer 11 (i.e., not by usual heatingwith the laser light) but by multiphoton absorption. The forming of amolten processed region by multiphoton absorption is disclosed, forexample, in “Silicon Processing Characteristic Evaluation by PicosecondPulse Laser”, Preprints of the National Meetings of Japan WeldingSociety, Vol. 66 (April, 2000), pp. 72-73.

A fracture is generated in a silicon wafer from a starting point regionfor cutting formed by a molten processed region, acting as a startpoint, in a cross-sectional direction, and reaches the front and rearfaces of the silicon wafer, whereby the silicon wafer is cut. Thefracture reaching the front and rear faces of the silicon wafer may grownaturally or as a force is applied to the silicon wafer. The fracturenaturally growing from the starting point region for cutting to thefront and rear faces of the silicon wafer encompasses a case where thefracture grows from a state where the molten processed region formingthe starting point region for cutting is molten and a case where thefracture grows when the molten processed region forming the startingpoint region for cutting is re-solidified from the molten state. Ineither case, the molten processed region is formed only within thesilicon wafer, and thus is present only within the cut section aftercutting as shown in FIG. 12. When a starting point region for cutting isthus formed within the object by a molten processed region, unnecessaryfractures deviating from a starting point region for cutting line areharder to occur at the time of cleaving, whereby cleavage controlbecomes easier.

(3) Case where the Modified Region is Formed by a Molten ProcessedRegion and a Microcavity

An object to be processed (e.g., semiconductor material such as silicon)is irradiated with laser light while locating a converging point withinthe object under a condition with a field intensity of at least 1×10⁸(W/cm²) at the converging point and a pulse width of 1 μs or less. Thismay form a molten processed region and a microcavity within the object.The upper limit of field intensity is 1×10¹² (W/cm²), for example. Thepulse width is preferably 1 ns to 200 ns, for example.

When laser light L is incident on a silicon wafer 11 from its front face3 side as shown in FIG. 14, a microcavity 14 is formed on the rear face21 side of the molten processed region 13. The molten processed region13 and the microcavity 14 are separated from each other in FIG. 14, butmay be formed continuously as well. Namely, when the molten processedregion 13 and the microcavity 14 are formed as a pair by multiphotonabsorption, the microcavity 14 is formed on the opposite side of themolten processed region 13 from the laser light entrance surface in thesilicon wafer 11.

It is not completely clear by what principle the microcavity 14 is thusformed so as to correspond to each molten processed region 13 formed bygenerating multiphoton absorption within the silicon wafer 11 bytransmitting the laser light L therethrough. Two hypotheses assumed bythe inventors concerning the principle by which the molten processedregion 13 and the microcavity 14 are formed as a pair will now beexplained.

The first hypothesis assumed by the inventors is as follows. Namely,when the silicon wafer 11 is irradiated with the laser light L focusedat a converging point P within the silicon wafer 111 as shown in FIG.15, the molten processed region 13 is formed near the converging pointP. Conventionally, light components in the center part of the laserlight L emitted from a laser light source (light componentscorresponding to L4 and L5 in FIG. 15) have been used as the laser lightL. This aims at using the center part of a Gaussian distribution of thelaser light L.

The inventors have tried to expand the laser light L in order torestrain the laser light L from affecting the front face 3 of thesilicon wafer 11. In one technique therefor, the laser light L emittedfrom the laser light source is expanded by a predetermined opticalsystem, so as to widen the skirt of the Gaussian distribution, therebyrelatively raising the laser intensity of light components in aperipheral part of the laser light L (those corresponding to L1 to L3and L6 to L8 in FIG. 15). When thus expanded laser light L istransmitted through the silicon wafer 11, the molten processed region 13is formed near the converging point P, and the microcavity 14 is formedat a part corresponding to the molten processed region 13 as explainedabove. Namely, the molten processed region 13 and the microcavity 14 areformed at respective positions on the optical axis (dash-dot line inFIG. 15) of the laser light L. The position at which the microcavity 14is formed corresponds to a part where light components in the peripheralpart of the laser light L (those corresponding to L1 to L3 and L6 to L8in FIG. 15) are theoretically converged.

The spherical aberration of a lens converging the laser light L seems tocause light components in the center part of the laser light L (thosecorresponding to L4 and L5 in FIG. 15) and light components in theperipheral part of the laser light L (those corresponding to L1 to L3and L6 to L8 in FIG. 15) to converge at respective parts different fromeach other in the thickness direction of the silicon wafer 11 as in theforegoing. The first hypothesis assumed by the inventors lies in thatthe difference in converging positions may have some effects.

The second hypothesis assumed by the inventors lies in that, since thepart where light components in the peripheral part of the laser light L(those corresponding to L1 to L3 and L6 to L8 in FIG. 15) are convergedis a theoretical laser-converging point, the light intensity is so highin this part that minute structural changes occur, thereby forming themicrocavity 14 whose surroundings do not substantially change theircrystal structure, whereas the part formed with the molten processedregion 13 is thermally affected so much that it is simply molten andre-solidified.

Here, the molten processed region 13 is as stated in (2) mentionedabove, whereas the microcavity 14 is one whose periphery does notsubstantially change its crystal structure. When the silicon wafer 11has a silicon monocrystal structure, the periphery of the microcavity 14mostly keeps the silicon monocrystal structure.

By an experiment, the inventors verified that the molten processedregion 13 and microcavity 14 were formed within the silicon wafer 11.The following are conditions of the experiment.

(A) Object to be Processed: Silicon Wafer (with a Thickness of 100 μm)

(B) Laser

-   -   light source: semiconductor laser pumping Nd:YAG laser    -   wavelength: 1064 nm    -   repetition frequency: 40 kHz    -   pulse width: 30 ns    -   pulse pitch: 7 μm    -   processing depth: 8 μm    -   pulse energy: 50 μJ/pulse

(C) Condenser Lens

-   -   N.A.: 0.55

(D) Moving Rate of the Mount Table Mounting the Object: 280 mm/sec

FIG. 16 is a view showing photographs of a cut section of the siliconwafer 11 cut by laser processing under the above-mentioned conditions.In FIG. 16, (a) and (b) are photographs showing the same cut section ondifferent scales. As depicted, within the silicon wafer 11, pairs ofmolten processed regions 13 and microcavities 14, each pair being formedby irradiation with one pulse of laser light L, are positioned at apredetermined pitch along the cross section (i.e., along a line to cut).

Each molten processed region 13 in the cut section shown in FIG. 16 hasa width of about 13 μm in the thickness direction of the silicon wafer11 (the vertical direction in the drawing) and a width of about 3 μm inthe moving direction of laser light L (the horizontal direction in thedrawing). Each microcavity 14 has a width of about 7 μm in the thicknessdirection of the silicon wafer 11 and a width of about 1.3 μm in themoving direction of laser light L. The gap between the molten processedregion 13 and microcavity 14 is about 1.2 μm.

(4) Case where the Modified Region is a Refractive Index Changed Region

An object to be processed (e.g., glass) is irradiated with laser lightwhile locating a converging point within the object under a conditionwith a field intensity of at least 1×10⁸ (W/cm²) at the converging pointand a pulse width of 1 ns or less. When multiphoton absorption isgenerated within the object with a very short pulse width, the energycaused by multiphoton absorption is not converted into thermal energy,whereby an eternal structure change such as ion valence change,crystallization, or orientation polarization is induced within theobject, thus forming a refractive index change region. The upper limitof field intensity is 1×10¹² (W/cm²), for example. The pulse width ispreferably 1 ns or less, for example, more preferably 1 ps or less. Theforming of a refractive index change region by multiphoton absorption isdisclosed, for example, in “Forming of Photoinduced Structure withinGlass by Femtosecond Laser Irradiation”, Proceedings of the 42nd LaserMaterials Processing Conference (November, 1997), pp. 105-111.

The cases (1) to (4) are explained in the foregoing as a modified regionformed by multiphoton absorption. A starting point region for cuttingmay be formed as follows while taking account of the crystal structureof a wafer-like object to be processed and its cleavage characteristic,whereby the object can be cut with a favorable precision by a smallerforce from the starting point region for cutting acting as a startpoint.

Namely, in the case of a substrate made of a monocrystal semiconductorhaving a diamond structure such as silicon, it will be preferred if astarting point region for cutting is formed in a direction extendingalong a (111) plane (first cleavage plane) or a (110) plane (secondcleavage plane). In the case of a substrate made of a group III-Vcompound semiconductor of sphalerite structure such as GaAs, it will bepreferred if a starting point region for cutting is formed in adirection extending along a (110) plane. In the case of a substratehaving a crystal structure of hexagonal system such as sapphire (Al₂O₃),it will be preferred if a starting point region for cutting is formed ina direction extending along a (1120) plane (A plane) or a (1100) plane(M plane) while using a (0001) plane (C plane) as a principal plane.

When the substrate is formed with an orientation flat in a direction tobe formed with the above-mentioned starting point region for cutting(e.g., a direction extending along a (111) plane in a monocrystalsilicon substrate) or a direction orthogonal to the direction to beformed therewith, the starting point region for cutting extending in thedirection to be formed with the starting point region for cutting can beformed easily and accurately with reference to the orientation flat.

The preferred embodiment of the present invention will now be explained.FIG. 17 is a plan view of the object to be processed in the laserprocessing method in accordance with this embodiment, whereas FIG. 18 isa sectional view of a part of the object taken along the lineXVIII-XVIII of FIG. 17. Referring to FIG. 17, the object to be processed1, which is a wafer, has a flat, substantially disk-like form. Aplurality of lines to cut 5 (grid-like lines to cut) intersecting eachother in longitudinal and lateral directions are set on the front faceof the object 1. The lines to cut 5 are virtual lines assumed forcutting the object 1 into a plurality of chip-like parts.

As shown in FIGS. 17 and 18, the object 1 comprises a substrate 4 havinga thickness of 30 μm to 150 μm made of silicon, and a multilayer part16, formed on the front face 4 a of the substrate 4, including aplurality of functional devices 15. Each functional device 15 has aninterlayer insulating film 17 a laminated on the front face 4 a of thesubstrate 4, a wiring layer 19 a formed on the interlayer insulatingfilm 17 a, an interlayer insulating film 17 b laminated on theinterlayer insulating film 17 a so as to cover the wiring layer 19 a,and a wiring layer 19 b formed on the interlayer insulating film 17 b.The wiring layer 19 a and the substrate 4 are electrically connected toeach other by a conductive plug 20 a penetrating through the interlayerinsulating film 17 a, whereas the wiring layers 19 a and 19 b areelectrically connected to each other by a conductive plug 20 bpenetrating through the interlayer insulating film 17 b.

While a number of functional devices 15 are formed like a matrix indirections parallel and perpendicular to an orientation flat 6 of thesubstrate 4, the interlayer insulating films 17 a, 17 b are formedbetween the functional devices 15, 15 adjacent to each other so as tocover the front face 4 a of the substrate 4 as a whole.

Thus configured object 1 is cut into the functional devices 15 asfollows. First, as shown in FIG. 19( a), a protective tape (protectivemember) 22 is attached to the front face 16 a of the multilayer part 16so as to cover the functional devices 15. Subsequently, as shown in FIG.19( b), the object 1 is fixed onto a mount table (not depicted) of alaser processing apparatus such that the rear face 4 b of the substrate4 faces up. Here, the protective tape 22 prevents the multilayer part 16from coming into direct contact with the mount table, whereby eachfunctional device 15 can be protected.

Then, lines to cut 5 are set like grids (see broken lines in FIG. 17) soas to pass between the functional devices 15, 15 adjacent to each other,and the substrate 4 is irradiated with laser light L under a conditiongenerating multiphoton absorption, while using the rear face 4 b of thesubstrate 4 as a laser light entrance surface, locating a convergingpoint P within the substrate 4, and moving the mount table so as to scanthe light-converging point P along the lines to cut 5. This forms amodified region 7 to become a starting point region for cutting withinthe substrate 4 such that the distance between the position of itsfront-side end part 7 a and the front face 4 a of the substrate 4(referring to the distance in the thickness direction of the substrate 4unless otherwise stated) is 3 μm to 40 μm. Forming the modified region 7under such a condition will generate a fracture 24 reaching the frontface 4 a of the substrate 4 from the front-side end part 7 a of themodified region 7.

Since the rear face 4 b of the substrate 4 is used as the laser lightentrance surface when irradiating the object 1 with the laser light L,the modified region 7 can be formed reliably within the substrate 4along the lines to cut 5 even when a member (e.g., TEG) reflecting thelaser light L exists on the lines to cut 5 of the multilayer part 16.Since the substrate 4 is a semiconductor substrate made of silicon, themodified region 7 is a molten processed region 13. Here, the modifiedregion 7 is formed by one row per line to cut 5.

After generating the fracture 24 by forming the modified region 7, anexpandable tape (expandable member) 23 is attached to the rear face 4 bof the substrate 4 as shown in FIG. 20( a). Subsequently, the protectivetape 22 is irradiated with UV rays as shown in FIG. 20( b), so as tolower its adhesive force, and then is peeled off from the front face 16a of the multilayer part 16 as shown in FIG. 21( a).

After peeling off the protective tape 22, the expandable tape 23 isexpanded as shown in FIG. 21( b), so as to cause a break from themodified region 7 acting as a start point, thereby cutting the substrate4 and multilayer part 16 along the lines to cut 5 and separatingsemiconductor chips 25 obtained by cutting from each other. This canyield the semiconductor chips 25 each comprising the substrate 4 and themultilayer part 16, formed on the front face 4 a of the substrate 4,including the functional device 15, whereas the modified region 7 isformed on a side face 4 c of the substrate 4 such that the distancebetween the position of the front-side end part 7 a and the front face 4a of the substrate 4 is 3 μm to 40 μm.

As explained in the foregoing, the above-mentioned laser processingmethod irradiates the substrate 4 with the laser light L while using therear face 4 b thereof as a laser light entrance surface in a state wherethe protective tape 22 is attached to the front face 16 a of themultilayer part 16, so as to form the modified region 7 within thesubstrate 4 along the lines to cut 5, thereby generating the fracture 24reaching the front face 4 a of the substrate 4 from the front-side endpart 7 a of the modified region 7. When the expandable tape 23 isattached to the rear face 4 b of the substrate 4 and expanded in thestate where such a fracture 24 is generated, not only the substrate 4but also the multilayer part 16, i.e., the interlayer insulating films17 a, 17 b, can be cut with a favorable precision along the lines to cut5. Namely, in the semiconductor chip 25 obtained by cutting, the endpart 16 c of the multilayer part 16 corresponding to the side face 4 cof the substrate 4 formed with the modified region 7 is cut with a highprecision as shown in FIG. 21( b).

The substrate 4 has a thickness of 30 μm to 150 μm in theabove-mentioned object 1. When the substrate 4 has a thickness of 30 μmto 150 μm as such, not only the multilayer part 16 but also thesubstrate 4 can be cut with a higher precision from one row of modifiedregion 7 acting as a start point.

Though the above-mentioned laser processing method generates thefracture 24 reaching the front face 4 a of the substrate 4 from thefront-side end part 7 a of the modified region 7 by forming the modifiedregion 7 along the lines to cut 5, a fracture 24 reaching the inside ofthe multilayer part 16 from the front-side end part 7 a of the modifiedregion 7 may be generated as shown in FIG. 22, and a fracture 24reaching the front face 16 a of the multilayer part 16 from thefront-side end part 7 a of the modified region 7 may be generated asshown in FIG. 23. Namely, when cutting the object 1 comprising thesubstrate 4 and the multilayer part 16, formed on the front face 4 a ofthe substrate 4, including the functional device 15, the multilayer part16 can be cut with a high precision in particular if the fracture 24reaching at least the front face 4 a of the substrate 4 from thefront-side end part 7 a of the modified region 7 is generated by formingthe modified region 7 within the substrate 4 along the lines to cut 5.

Though the above-mentioned laser processing method forms the modifiedregion 7 within the substrate 4 such that the distance between theposition of the front-side end part 7 a and the front face 4 a of thesubstrate 4 is 3 μm to 40 μm, the modified region 7 may be formed suchthat the center of the modified region 7 is positioned closer to thefront face 4 a of the substrate 4 than is the center of the substrate 4.Thus forming the modified region 7 can make it easier to generate thefracture 24 reaching at least the front face 4 a of the substrate 4 fromthe front-side end part 7 a of the modified region 7.

Reasons why the multilayer part 16 can be cut with a high precision whenthe fracture 24 reaching at least the front face 4 a of the substrate 4from the front-side end part 7 a of the modified region 7 is generatedwill now be explained. Here, it is assumed that a low dielectricconstant film (low-k film) is laminated as the multilayer part 16 on thefront face 4 a of the substrate 4 made of silicon.

(1) Case where the Fracture 24 Reaching the Front Face 4 a of theSubstrate 4 from the Front-Side End Part 7 a of the Modified Region 7 isGenerated

FIG. 24 is a sectional view of a part of the object 1 for explaining thefirst reason why the low dielectric constant film 26 can be cut with ahigh precision when the fracture 24 reaching the front face 4 a of thesubstrate 4 from the front-side end part 7 a of the modified region 7 isgenerated.

When the expandable tape 23 is expanded in the state where the fracture24 reaching the rear face 4 b of the substrate 4 from the rear-side endpart 7 b of the modified region 7 is generated as shown in FIG. 24( a),the fracture 24 extends toward the front face 4 a of the substrate 4very smoothly. Therefore, the state where the fracture 24 reaching therear face 4 b of the substrate 4 from the rear-side end part 7 b of themodified region 7 is generated can be construed as a state where thesubstrate 4 is easy to cut.

On the other hand, the state where the fracture 24 reaching the frontface 4 a of the substrate 4 from the front-side end part 7 a of themodified region 7 as shown in FIG. 24( b) can be construed as a statewhere the substrate 4 is harder to cut than in the state where thefracture 24 reaching the rear face 4 b of the substrate 4 from therear-side end part 7 b of the modified region 7 is generated.

When the expandable tape 23 is expanded in the state where the fracture24 reaching the front face 4 a of the substrate 4 from the front-sideend part 7 a of the modified region 7, i.e., in the state where thesubstrate 4 is hard to cut, the substrate 4 is not cut gradually as theexpandable tape 23 expands, but at once. This seems to prevent the lowdielectric constant film 26, which has a low mechanical strength and aproperty of being harder to blend with other materials in general so asto be prone to tear and peel off, from tearing and peeling off, and makeit possible to cut the low dielectric constant film 26 with a highprecision together with the substrate 4.

FIG. 25 is a sectional view of a part of the object 1 for explaining thesecond reason why the low dielectric constant film 26 can be cut with ahigh precision when the fracture 24 reaching the front face 4 a of thesubstrate 4 from the front-side end part 7 a of the modified region 7 isgenerated.

When the expandable tape 23 is expanded in the state where the fracture24 reaching the rear face 4 b of the substrate 4 from the rear-side endpart 7 b of the modified region 7 is generated as shown in FIG. 25( a),the substrate 4 is gradually cut as the expandable tape 23 is expanded.Therefore, at the time when the fracture 24 reaches the low dielectricconstant film 26, the low dielectric constant film 26 warps in thevalley-folding direction and tears in this state.

If the expandable tape 23 is expanded in the state where the fracture 24reaching the front face 4 a of the substrate 4 from the front-side endpart 7 a of the modified region 7 is generated as shown in FIG. 25( b),on the other hand, the substrate 4 will be cut at once at the time whena predetermined expansive force acts on the substrate 4. This preventsthe low dielectric constant film 26 from tearing in the state warped inthe valley-folding direction.

Therefore, the state where the fracture 24 reaching the front face 4 aof the substrate 4 from the front-side end part 7 a of the modifiedregion 7 is generated seems to be able to cut the low dielectricconstant film 26 with a higher precision together with the substrate 4than the state where the fracture 24 reaching the rear face 4 b of thesubstrate 4 from the rear-side end part 7 b of the modified region 7 isgenerated.

(2) Case where the Fracture 24 Reaching the Front Face 26 a of the LowDielectric Constant Film 26 from the Front-Side End Part 7 a of theModified Region 7 is Generated

FIG. 26 is a sectional view of a part of the object 1 for explaining areason why the low dielectric constant film 26 can be cut with a highprecision when the fracture 24 reaching the front face 26 a of the lowdielectric constant film 26 from the front-side end part 7 a of themodified region 7 is generated. As shown in this drawing, the lowdielectric constant film 26 is cut at the time when the modified region7 is formed within the substrate 4 along the lines to cut 5 in thiscase. Therefore, this seems to be able to cut the low dielectricconstant film 26 while preventing it from tearing and peeling off.

The results of cutting in the cases of (1) and (2) mentioned above areas follows. As shown in FIG. 27, both of the cases where the fracture 24reached the front face 4 a of the substrate 4 and where the fracture 24reached the front face 26 a of the low dielectric constant film 26 wereable to cut the low dielectric constant film 26 with a very highprecision (see the photographs in the lower part). Also, they were ableto suppress the tear of the low dielectric constant film 26 to less than5 μm even in the part where Al pads 27 are formed on the lines to cut 5(see the photographs in the middle part).

Here, the front-side end part distance of the modified region 7 refersto the distance between the position of the front-side end part of themodified region 7 and the front face 4 a of the substrate 4, whereas therear-side end part distance of the modified region 7 refers to thedistance between the position of the rear-side end part 7 b of themodified region 7 and the rear face 4 b of the substrate 4. The width ofthe modified region 7 refers to the distance between the position of thefront-side end part 7 a and the position of the rear-side end part 7 bin the modified region 7. The position of the front-side end part 7 a ofthe modified region 7 refers to “the average position in the thicknessdirection of the substrate 4” of “the end part facing the front face 4 aof the substrate 4” of the modified region 7 formed along the lines tocut 5, whereas the position of the rear-side end part 7 b of themodified region 7 refers to “the average position in the thicknessdirection of the substrate 4” of “the end part facing the rear face 4 bof the substrate 4” of the modified region 7 formed along the lines tocut 5 (see the photographs in the upper part of FIG. 27).

The relationship between the distance from the position of thefront-side end part 7 a of the modified region 7 to the front face 4 aof the substrate 4 and the state of the substrate 4 will now beexplained.

FIGS. 28 to 31 are charts showing “the relationships between thedistance from the position of the front-side end part 7 a of themodified region 7 to the front face 4 a of the substrate 4 and the stateof the substrate 4” for the substrates 4 having thicknesses of 30 μm, 50μm, 100 μm, and 150 μm, respectively.

In each chart, (a) is a case where the converging point P of laser lightL is scanned once along the lines to cut 5, whereas (b) is a case wherethe converging point P of laser light L is scanned twice along the linesto cut 5. State DM of the substrate 4 refers to a state where the frontface 4 a of the substrate 4 is dotted with damages, whereas state FL ofthe substrate 4 refers to a state where the fracture 24 has reached thefront face 4 a of the substrate 4. State ST of the substrate 4 refers toa case where no changes appear in any of the front face 4 a and rearface 4 b of the substrate 4, whereas state HC of the substrate 4 refersto a case where the fracture 24 has reached the rear face 4 b of thesubstrate 4.

A bare wafer made of silicon was used as the substrate 4 when verifying“the relationship between the distance from the position of thefront-side end part 7 a of the modified region 7 to the front face 4 aof the substrate 4 and the state of the substrate 4”. The irradiationconditions for laser light L along the lines to cut 5 are as follows:

Repetition frequency: 80 kHz

Pulse width: 150 ns

Pulse energy: 15 μJ

Processing speed (moving speed of the converging point P with respect tothe substrate 4): 300 mm/sec

As is clear from FIGS. 28( a) to 31(a), the fracture 24 reaching atleast the front face 4 a of the substrate 4 from the front-side end part7 a of the modified region 7 can reliably be generated (state FL of thesubstrate 4) when the modified region 7 is formed such that the distancebetween the position of the front-side end part 7 a of the modifiedregion 7 and the front face 4 a of the substrate 4 is 3 μm to 35 μm inthe case where the laser light L is irradiated once along the lines tocut 5.

As is clear from FIGS. 28( b) to 31(b), the fracture 24 reaching atleast the front face 4 a of the substrate 4 from the front-side end part7 a of the modified region 7 can reliably be generated (state FL of thesubstrate 4) when the modified region 7 is formed such that the distancebetween the position of the front-side end part 7 a of the modifiedregion 7 and the front face 4 a of the substrate 4 is 3 μm to 40 μm inthe case where the laser light L is irradiated twice along the lines tocut 5.

The present invention is not limited to the above-mentioned embodiment.

For example, as shown in FIG. 32, the modified region 7 may be formedwithin the substrate 4 along the lines to cut 5 such that the front-sideend part 7 a of the modified region 7 extends like a streak to the frontface 4 a of the substrate 4 ((a) and (b) in FIG. 32 showing photographsof the same cut section on different scales). Thus forming the modifiedregion 7 generates the fracture 24 reaching at least the front face 4 aof the substrate 4 from the front-side end part 7 a of the modifiedregion 7. When the expandable tape 23 is attached to the rear face 4 bof the substrate 4 and expanded in the state where such a fracture 24 isgenerated, not only the substrate 4 but also the multilayer part 16 (thelow dielectric constant film 26 in FIG. 32) in particular can be cutwith a favorable precision along the lines to cut 5. Here, thefront-side end part 7 a of the modified region 7 is likely to extendlike a streak to the front face 4 a of the substrate 4 when a member(e.g., metal wire or metal pad) reflecting the laser light L exists onthe lines to cut 5 in the multilayer part 16.

Though the above-mentioned embodiment is a case where the moltenprocessed region 13 is formed as the modified region 7 within asemiconductor substrate such as silicon wafer, a molten processed region13 and a microcavity 14 positioned closer to the front face 4 a of thesubstrate 4 than is the molten processed region 13 may be formed as themodified region 7. Thus forming the microcavity 14 positioned closer tothe front face 4 a of the substrate 4 than is the molten processedregion 13 improves the rectilinearity of the fracture 24 reaching atleast the front face 4 a of the substrate 4, whereby the multilayer part16 in particular can be cut with a higher precision along the lines tocut 5.

Though the above-mentioned embodiment is a case where the modifiedregion 7 is formed by generating multiphoton absorption within thesubstrate 4, there is a case where the modified region 7 can be formedby generating light absorption equivalent to multiphoton absorptionwithin the substrate 4.

Examples of the multilayer part formed on the lines to cut includeorganic and inorganic insulating films; their composite films; lowdielectric constant films; conductive films such as TEG, metal wires,and electrodes; and those formed with at least one layer of them.

INDUSTRIAL APPLICABILITY

When cutting a substrate formed with a multilayer part including aplurality of functional devices, the present invention makes it possibleto cut the multilayer part with a high precision in particular.

1. A laser processing method of irradiating a substrate having a frontface formed with a multilayer part including a plurality of functionaldevices with laser light while locating a converging point within thesubstrate, so as to form a modified region to become a starting pointregion for cutting within the substrate along a line to cut in thesubstrate; wherein the modified region is formed such as to generate afracture reaching at least the front face of the substrate from afront-side end part of the modified region.
 2. A laser processing methodaccording to claim 1, wherein the modified region is formed such as togenerate the fracture reaching the inside of the multilayer part fromthe front-side end part of the modified region.
 3. A laser processingmethod according to claim 1, wherein the modified region is formed suchas to generate the fracture reaching the front face of the multilayerpart from the front-side end part of the modified region. 4-12.(canceled)
 13. A laser processing method according to claim 1, whereinthe substrate is a semiconductor substrate, and wherein the modifiedregion includes a molten processed region.
 14. A laser processing methodaccording to claim 1, wherein the substrate is a semiconductorsubstrate, and wherein the modified region includes a molten processedregion and a microcavity positioned closer to the front face of thesubstrate than is the molten processed region.
 15. A laser processingmethod according to claim 1, wherein the substrate has a thickness of 30μm to 150 μm.
 16. A laser processing method according to claim 1,wherein the substrate and multilayer part are cut along the line to cutafter forming the modified region.
 17. A laser processing method ofirradiating a substrate having a front face formed with a multilayerpart including a plurality of functional devices with laser light whilelocating a converging point within the substrate, so as to form amodified region to become a starting point region for cutting within thesubstrate along a line to cut in the substrate; wherein the modifiedregion is formed such that the front-side end part of the modifiedregion extends like a streak to the front face of the substrate.
 18. Alaser processing method according to claim 17, wherein the substrate isa semiconductor substrate, and wherein the modified region includes amolten processed region.
 19. A laser processing method according toclaim 17, wherein the substrate is a semiconductor substrate, andwherein the modified region includes a molten processed region and amicrocavity positioned closer to the front face of the substrate than isthe molten processed region.
 20. A laser processing method according toclaim 17, wherein the substrate has a thickness of 30 μm to 150 μm. 21.A laser processing method according to claim 17, wherein the substrateand multilayer part are cut along the line to cut after forming themodified region.
 22. A laser processing method of irradiating asubstrate having a front face formed with a multilayer part including aplurality of functional devices with laser light while locating aconverging point within the substrate, so as to form a modified regionto become a starting point region for cutting within the substrate alonga line to cut in the substrate; wherein the modified region is formedsuch that the distance between a position of a front-side end part ofthe modified region and the front face of the substrate is 3 μm to 40μm.
 23. A laser processing method according to claim 22, wherein themodified region is formed such that the distance between the position ofthe front-side end part of the modified region and the front face of thesubstrate is 3 μm to 35 μm when the laser light is irradiated once alongthe line to cut.
 24. A laser processing method according to claim 22,wherein the front-side end part of the modified region and the frontface of the substrate is 3 μm to 40 μm when the laser light isirradiated a plurality of times along the line to cut.
 25. Asemiconductor chip comprising a substrate and a multilayer part, formedon a front face of the substrate, including a functional device, a sideface of the substrate being formed with a modified region; wherein themodified region is formed such that the distance between a position of afront-side end part of the modified region and the front face of thesubstrate is 3 μm to 40 μm.